Mercury cell having rotating anode

ABSTRACT

A NEW ROTARY ANODE, MERCURY CATHODE ELECTROLYTIC CELL FOR THE MANUFACTURE OF CHLORINE AND CAUSTIC ALKALIES FROM AQUEOUS ALKALI METAL CHLORIDE BRINES PROVIDES THE ADVANTAGES OF HIGH CURRENT DENSITY, HIGH CURRENT EFFICIENCY, LOW FLOOR SPACE AND LOW COST PER TON OF PRODUCT.

May 25, 1971 R COOPER 3,580,833

MERCURY CELL HAVING ROTATING ANODE Filed April 30, 1969 2 Sheets-Sheet lFIG-1 INVENTOR R0) M COOPER 415" 4 BY 1771%"1 V AGENT United StatesPatent l 3,580,833 MERCURY CELL HAVING ROTATING ANODE Roy M. Cooper,Groton, Conn., assignor to Olin Mathieson Chemical Corporation FiledApr. 30, 1969, Ser. No. 820,459 Int. Cl. C23b 5/68; C01d 1/08 US. Cl.204212 8 Claims ABSTRACT OF THE DISCLOSURE A new rotary anode, mercurycathode electrolytic cell for the manufacture of chlorine and causticalkalies from aqueous alkali metal chloride brines provides theadvantages of high current density, high current efiiciency, low floorspace and low cost per ton of product.

This invention relates to novel structures suitable for use inelectrolytic cells, particularly cells of the liquid cathode type and,more particularly, in mercury cathode electrolytic cells. The use of thestructures of this invention in other cells of similar construction isalso contemplated.

Horizontal mercury cells usually consist of an enclosed, elongatedtrough sloping slightly towards one end. The cathode is a flowing layerof mercury which is introduced at the higher end of the cell and flowsby gravity along the bottom of the cell toward the lower end. The anodesare generally composed of rectangular blocks of graphite suspended fromconductive lead-ins, for example, graphite or protected copper tubes orrods, in such a manner that the bottom of the graphite anode is spaced ashort distance above the flowing mercury cathode. The bottom and sidesof the trough are generally steel with a corrosion resistant hard rubberlining on the sides and under the cover. Concrete, stone or othernon-conducting material may also be used for the sides. The lining maycomprise concrete which is further coated with resin, or it may benatural stone set in a concrete lining.

More recently anodes of other materials than graphite have come intouse. Titanium anodes, with one or more of the platinum group metals ortheir oxides coated on the side of the anode facing the cathode, areespecially advantageous.

When the anodes are titanium coated with a platinum group metal, alloyor oxide thereof, the anodes are fabricated in any suitable form, forexample, a sheet or expanded mesh, suitably reinforced as necessary.

In the operation of this type of cell, the electrolyte is an aqueoussolution of any electrolyte which upon electrolytic decomposition willgive the products desired. The electrolyte is introudced at the upperend of the cell and flows toward the lower end of the cell. Directcurrent passes through the solution between the anodes and the mercurycathode. When sodium chloride is the electrolyte, chlorine is formed atthe anodes and passes to the top of the cell and out through an openingin the cell cover. Sodium is formed at the cathode as an amalgam withthe mercury cathode. The sodium amalgam is withdrawn at the lower end ofthe cell, cycled to a decomposer packed with graphite where it iscontacted with water to form sodium hydroxide, hydrogen and mercury. Themercury is recycled to the cell for reuse as the cathode. Brines ofother electrolytes, particularly potassium chloride and lithiumchloride, are also suitably electrolyzed in such cells.

In these cells, chlorine bubbles adhering to the anode surfaces reducethe active surface in contact with the brine and contributesubstantially to the total resistance in the cell. This increases thevoltage required to force the electric current to flow through the cellespecially at high ice current densities. Various anode designs areknown in the art to facilitate the removal of bubbles and collection ofthe gas, including drill holes, channels and slots variously arranged.See, for example, US. Pats. 3,062,733; 3,174,- 923 and 3,268,427.

The mercury cell of the present invention is generally cylindrical andhas a rotating anode. This structure materially improves the dischargeof chlorine from the active face of the anode, improves the circulationof brine in the inter-electrode space and accelerates the flow ofmercury and/or amalgam over the bottom of the cell. In addition, theanode is readily adjustable to maintain the interelectrode distance atoptimum.

The mercury cathode cell of the present invention combines a cover and abottom which are generally circular in shape and which are joined bysides generally cylindrical in shape, a mercury cathode disposed on saidbottom and an anode generally circular in shape disposed between saidbottom and said cover, inlet means for strong brine and mercury andoutlet means for weak brine and for chlorine and amalgam products; acircumferential well around said bottom for amalgam accumulation anddischarge; cathode current-carrying means attached to said bottom;rotatable axial anode support means attached perpendicularly to saidanode and extending outside said cell; and anode current-carrying means,drive means and bearing means attached to said support means.

This invention, in another aspect, contemplates the improvement in themethod of electrolysis of aqueous alkali metal chloride brines in a cellhaving a flowing mercury cathode and an anode parallel thereto andspaced therefrom, of maintaining the spacing between the anode andcathode at less than A inch and rotating said anode about its axis. In aparticularly advantageous form of the method of the invention, the anodeis rotated to propel the brine relative to the anode at a weight rate offlow such that its Reynolds number is at least 4000.

The anode support means of the cell of this invention is rotatable andis axially arranged with reference to the anode. The anode support meansis suitably a shaft which may be tubular or solid. Advantageously, it isfabricated of titanium or alloys thereof which have superior resistanceto the wet chlorine gas and chlorinated brine to which it is exposed.Use of titanium eliminates the necessity of providing sealing devices toprevent contact of these corrosive materials with the shaft. However,graphite or other metals, alloys or non-metals of satisfactory corrosionresistance and structural strength are suitable.

Suitable anodes for use in this invention are composed of cylindricalblocks of graphite. The lower surface facing the cathode isappropriately cut with concentric circular channels and advantageously aplurality of drill holes are provided connecting said channels with theupper surface of the anode. These channels and drill holes facilitatethe escape of chlorine gas from the anode-cathode gap. Suitably rotarygraphite anodes are up to about 12" thick when new and are used untilthey are only 1 or 2 inches thick when the danger of breakage issufiicient to justify replacement. Larger anodes are suitably built upof segments of graphite.

Anodes of materials other than graphite are also suitable in theserotating anode mercury cathode cells, particularly titanium anodeshaving a thin coating over at least part of their surface of a platinumgroup metal or oxide thereof. The term, platinum group metal as used inthis specification and claims means an element of the group consistingof ruthenium, rhodium, palladium, osmium, iridium and platinum andalloysthereof. The term titanium includes alloys consisting essentially oftitanium. These coated titanium anodes have the advantage ofsubstantially complete resistance to corrosion and they thereforerequire little or no adjustment of the interelectrode spacing. Forrotary anodes, they are particularly advantageous for their light weightwhich permits fabrication of large diameter anodes at low weight and lowbearing loads. Suitably coated titanium anodes are fabricated of solidsheet or expanded mesh with reinforcing ribs or vanes to ensurepresentation of a surface substantially parallel to the cathode surface.

The anode is suitably supported from above the cover of the cell or frombelow the cell. When support is from above, the anode current-carryingmeans are conveniently arranged on top of the cell cover. Rotary drivemeans, for example, gears or belts, and means for adjusting theelevation of the anode above the cathode are also suitably arrangedabove the cell cover. When support is from below the cell, severaladvantages appear:

(1) The cell cover and sides are entirely separate from the anodes andanode supports. The cover and sides are suitably constructed of verylight weight materials easily handled by a mobile crane, for example, acherry picker.

(2) Graphite anodes are not suspended from a post into the graphite andthe anodes have no tendency to break loose and drop into the mercurycathode.

(3) Intercell bus bar costs are reduced since all buses are below thecell.

(4) Anode seal costs are eliminated since there are no anode' seals.

(5) The cell is lighter and supporting structures are less expensive,especially using titanium anodes.

(6) Anode stub losses are radically reduced by supporting the anodesfrom below the cell.

(7) Electrical resistance is decreased because the current to the cellbottom and to the anodes is more uniformly distributed. Anodes supportedfrom below provide better electrical contact between titanium andgraphite.

In the preferred embodiment of the invention, the anode current-carryingmeans attached to the rotating axial anode support has rotatable andstationary elements, one of which has the form of an annulus andcontains an electrically conductive liquid. The other element has atleast one and preferably a plurality of dependent flanges partiallyimmersed in the electrically conductive liquid. Preferably thestationary element contains the liquid but alternatively the rotatableelement contains the liquid. Suitably the conductive liquid is aqueousalkali metal chloride brine or liquid metal, e.g., gallium or liquidalloys. Preferably, however, the conductive liquid is mercury. Thisprovides electrical contact with low voltage drop between movingmetallic surfaces and requires no adjustment because there is no wear.Other means for conducting the current to the rotating anode aregenerally unsatisfactory. Brushes or other sliding contacts to the anodesupport means and anode wear rapidly, become dirty and severely pit themoving surface of the anode support shaft. The mercury filled contactingdevice avoids these problems and provides advantageous contacting means.

Generally, in this cell, as in most modern commercial cells, a pressureslightly below atmospheric is maintained in the cell. Air leakage inwardis preferred to any leakage of chlorine out of the cell. For thisreason, gas-tight bearings are not required in the cell of the presentinvention.

In operation, according to this invention, brine inlet and outlet flows,brine preparation and purification, amalgam fiow to decomposers, recyclemercury flow, hydrogen and chlorine collection and treatment areconventional. Means for rotation of the anode are suitably hydraulic orelectric motors or other conventional means of rotation. Speed ofrotation of the anode is generally inversely related to its diameter,but the number of revolutions per minute is not adequate alone to defineadvantageous op erating conditions. The size of the anode, its bottomconfiguration, the flow of brine and the interelectrode spacing areadditional variable factors. All of these are taken 4 into account inrelating the operation to the Reynolds number which is a measure of theturbulence in the brine layer between the electrodes.

Reynolds number is defined and calculated in known manner as described,for example, in US. Pat. 2,836,551. With the rotating anode, thevelocity varies radially from zero at the center to a maximum at theperiphery. Reynolds numbers herein were calculated at the midpoint ofthe anode areas. Turbulent flow begins appreciably at Reynolds numbersof about 4000 and the voltage required to produce a given currentdensity begins to de crease. In the range of Reynolds numbers of 10,000to 20,000, the voltage decreases and apparently reaches a minimum atabout 20,000. For operation at this minimum voltage, the anode in thecell of this invention is rotated at a rate to produce Reynolds numbersof at least 4000 and preferably in the range of 10,000 to 20,000. Sincethe Reynolds number is a function of the radius of the anode, amongother factors, the r.p.m. of the anode is lower with anodes of largerradius and higher with anodes of smaller radius, other factors beingconstant. Operation in the advantageous range of Reynolds numberspermitlower voltages to produce higher anode current densities than in"horizontal mercury cells with stationary anodes.

EXAMPLE I A cell substantially of the design shown in FIG. 1, exceptthat the anode was not channeled or slotted, was operated at anodecurrent densities of 8, 10 and 12 amperes per square inch. Anode-cathodegap was 0.1875". The rotation of the 9 inch diameter anode was variedfrom 112 to 325 r.p.m. satisfactorily. The voltage at an anode currentdensity of 10 amperes per square inch was 4.4, current efiiciency 95.5%,cell gas chlorine was 97% and cell gas hydrogen was under 1%.

EXAMPLE II Using the cell of Example I having a solid 9 inch diameterrotating graphite anode and an interelectrode spacing of 0.1825 inch andmaintaining an anode current density of 4.75 amperes per square inch,the rotational speed of the anode was varied to show the voltagereduction possible at increasing Reynolds numbers. The data obtainedappear in Table 1.

TABLE 1 Reynolds Rpm number Voltage The Reynolds numbers were calculatedat the midpoint of the active anode area and represent an average overthe entire anode area. The data show the advantageous reduction ofvoltagedue to rotation of the anode while maintaining constant currentdensity. Turbulent flow of the electrolyte is shown at 76 r.p.m. andminimum voltage is achieved at about 300 r.p.m.

EXAMPLE IV The cell and rotating slotted anode of Example II was used tocompare this cell with commercial horizontal mercury cells identified asE8 and E-ll to show the advantage of the cell of this invention in termsof required floor space for given input of energy and therefore outputof product. The data are shown in Table 2.

TABLE 2 Anode Amps] current Total It. of density, Cell, load, floor Cellamps. /in. volts kiloarnps area E-8. 3. 63 4. 32 33. 167 4 3. 56 33. 5234 6 3. 75 33. 5 307 Rotating slotted anode cell. 8 3. 96 33. 5 368 104. 25 33. 5 414 12 4. 56 33. 5 459 13-11 4. 63 4. 16 100 295 4 3. 56 100295 6 3. 75 100 405 Rotating slotted anode cell... 8 3. 96 100 500 10 4.25 100 581 12 4. 56 100 658 The data of Table 2 show that the rotatinganode cell requires 30 to 50% less floor space than an E-S cell andoperates with 7 to 16% lower cell voltage at the same total load. Athigher cell loads, for example at the 100 kiloamperes used in the E-l-lcell, the rotating disc cell requires 41% less floor space and 5 to 14%lower cell voltage.

FIG. 1 herewith shows one embodiment of the mercury cell of theinvention in which a rotary graphite anode is supported above the cellcover. FIG. 2 shows a rotary titanium anode for use in a cell otherwisethe same as the cell of FIG. 1. FIG. 3 shows an embodiment of theinvention in which the rotary titanium anode is supported from below thecell.

More particularly, in FIG. 1, the cell generally consists of bottom 11,cover 12 and cylindrical sides 13. Mercury enters the cell at mercuryinlet 14, covers bottom 11 and collects in circumferential amalgam well15, flowing out of the cell via amalgam outlet 16. Brine flows into thecell via brine inlet 17 and out via brine outlet 18. Immersed in thebrine is anode 19 having circular channels 20 in its lower faceconnected to the upper surface of circular anode 19 by drill holes 21.Bolts 22 hold anode 19 to shaft 23 which is supported by collar 24 onbearing surface 25. Lower bearing 26 is provided in cover 12. The coveris bolted to sides 13 by bolts 27 and a chlorine gas outlet is providedat 28. Shaft 23 carries drive means, suitably pulley 29 and rotatingelement 30 of the current-carrying means. Stationary element 31 isattached to cover 12 and carries flexible anode bus 32 and block 33.Mercury 34 fills stationary element 31. Cathode current is supplied byflexible cathode bus 44 and block 45 attached to cell bottom 11.

FIG. 2 shows a rotary titanium anode for use in a cell otherwise thesame as FIG. 1. Titanium sheet or expanded mesh 35 bearing a thincoating of platinum on the bottom side forms the active surface of theanode. Vanes 36 support the titanium from ring 37. These parts,including shaft 23 are all advantageously fabricated of titanium.

FIG. 3 shows a rotary titanium anode supported from below the cell.Corresponding parts bear the same numbers as in FIGS. 1 and 2. Shaft 38rests in thrust bearing 39 suitably supported, for example, on concrete40. Shaft 38 carries drive means 29 and rotating element 30 of thecurrent-carrying means. Stationary element 31 carries anode bus 32 andblock 33 resting on concrete pier 41. Shaft 38 extends upward throughthe cell bottom 11 and is insulated therefrom by bearing 42 which alsoprevents outflow of mercury and brine. Mercury well 43 is formed byinsulating bearing 42 and cell bottom 11 and mercury inlet 14communicates with well 43. The mercury flows across the surface ofbottom 11 to amalgam well 15. Cathode current is supplied by flexiblecathode bus 44 and block 45 attached to cell bottom 11.

What is claimed is:

1. A mercury cathode cell for electrolysis of aqueous alkali metalchloride brines, comprising the combination of a cover and a bottomwhich are generally circular in shape and which are joined by sidesgenerally cylindrical in shape, a mercury cathode disposed on said bottom and an anode generally circular in shape disposed between saidbottom and said cover, inlet means for strong brine and mercury andoutlet means for weak brine and for chlorine and amalgam products; acircumferential well around said bottom for amalgam accumulation anddischarge; cathode current-carrying means attached to said bottom; saidanode having concentric circular channels in its lower surface facingsaid cathode and a plurality of drill holes connecting said channelswith the upper surface of said anode, rotatable axial anode supportmeans attached perpendicularly to said anode and extending outside saidcell; and anode currentcarrying means including stationary and rotatableelements in contact with an electrically conductive liquid contained inone of said elements, drive means and bearing means attached to saidsupport means.

2. Mercury cell as claimed in claim 1 in which said conductive liquid isa liquid metal.

3. Mercury cell as claimed in claim 2 in which said liquid metal ismercury.

4. Mercury cell as claimed in claim 1 in which said rotatable element isattached to said anode support means and depends into said electricallyconductive liquid contained in said stationary element.

5. Mercury cell as claimed in claim 4 in which said anode support meansextends above said cover and said stationary element is attached to saidcover.

6. Mercury cell as claimed in claim 1 in which said anode support meansextends below the bottom of said cell and sleeve means surround andinsulate said support means from said brine and said mercury cathode.

7. Mercury cell as claimed in claim 1 in which said anode is titaniumcoated over at least part of its surface with a platinum group metal oroxide thereof.

8. Mercury cell as claimed in claim 1 in which said bottom has an inletwhereby mercury is supplied to form said cathode.

References Cited UNITED STATES PATENTS 646,313 3/1900 Rhodin 2042123,409,519 11/1968 Gallone et al 20499 WINSTON A. DOUGLAS, PrimaryExaminer H. A. FEELEY, Assistant Examiner US. Cl. X.R. 20499, 250

